4 Answers
4

The Talmud says when you look into a prospective girl you should look into her brothers (Bava Basra 110a).

Rav Yaakov Kaminetzky is reported to have said that this no longer applies, as it was said in a time when girls stayed in the home and that was their primary sphere of influence. However, today a girl will be influenced by a much broader sphere and her brothers are not as strong of indicators.

However, the Maharal (Chiddushe Agados to Bava Basra 110a) understands that the nature of boys and girls who come from the same parents are different, and one can not see the traits of boys that come from this girls parents by seeing her, i.e. if you want to see what she would have been like had she been a boy, check out her brothers, as they come from the same womb. This would still apply today.

Shulchan Aruch Even Ha'Ezer 2:2 says not to marry someone from a family that is argumentative or accusational.

The first quality that comes to mind from the Torah is good Midos. Avraham did not want Yitzchok to marry the daughters of Canaan (Breishis 24:3 and Kli Yakar there) since he did not want Yitzchok to learn from their bad Midos. When Eliezer goes to find Rivka his test is if she is a Baalas Chesed, again checking out her Midos. (Breishis 24:14).

There are number of categories of people one may not marry, so one should make sure that the person one is planning on marrying isn't in any of these categories. These categories include non-Jews, close relatives, and married people. For some Jews they also include converts, mamzerim, divorcees, or others. (Sources include Rambam.)

a person should always try to marry a daughter of a "Talmid Chacham", and marry off his daughter to a "Talmid Chacham", if he did not find a daughter of a "Talmid Chacham" he should marry a daughter of a "Godol" of the generation, if he did not find a daughter of a "Godol" of the generation he should marry a daughter of a "leader of the gatherings" (i saw an explanation that this means the one who decides who gets kibudim (aliyois) in the shul), if he did not find a daughter of a "leader of the gatherings" he should marry a daughter of a "gabai tedoko", if he did not find a daughter of a "gabai tedoko" he should marry a daughter of a teacher of children. do not marry off your daughter to an "am hoorets".
[Hago: about their daughters it says "Cursed be he that lieth with an animal" (Deuteronomy 27 21) all this is concerning an "am hoorets" that is not scrupulous in mitsvois. it is a mitsva for a man to marry the daughter of his sister, their are those that say that (the mitsva is fulfilled) even the daughter of his brother]